1. Field of the Invention
The invention relates generally to a global positioning system (GPS) receiver and more particularly for a GPS receiver integrating a GPS signal for an “A” time period having “A” time segments and integrating the GPS signal for a “B” time period having “B” time segments, where the “A” time segments and the “B” time segments alternate, for signal acquisition at a low signal strength.
2. Description of the Prior Art
The global positioning system (GPS) is a system using GPS satellites for broadcasting GPS signals having information for determining location and time. Each GPS satellite broadcasts a GPS signal having 20 milliseconds (ms) GPS data bits modulated by a 1 ms pseudorandom noise (PRN) code having 1023 bits or chips. The PRN code for each GPS satellite is distinct, thereby enabling a GPS receiver to distinguish the GPS signal from one GPS satellite from the GPS signal from another GPS satellite. The 20 ms GPS data bits are organized into a frames of fifteen hundred bits. Each frame is subdivided into five subframes of three hundred bits each.
Typically, when the GPS receiver is first turned on, it knows its own approximate location, an approximate clock time, and almanac or ephemeris information for the locations-in-space of the GPS satellites as a function of clock time. The GPS receiver processes the approximate time, its approximate location, and the almanac or ephemeris information to determine which of the GPS satellites should be in-view and generates one or more GPS replica signals having carrier frequencies and a pseudorandom noise (PRN) codes matching the estimated Doppler frequencies and the PRN codes of one or more of the in-view GPS satellites. The GPS receiver correlates the carrier frequency, the PRN code, and the PRN code phase of the incoming GPS signal to the replica signals and then accumulates a correlation level. The process of correlation and accumulation may need to be repeated many times until a correlation level is found that exceeds a correlation threshold, thereby indicating GPS signal acquisition for the frequency, code, and code phase of the replica signals.
The incoming GPS signal has a low signal-to-noise ratio because of the spreading effect of the PRN code. The effect of the correlation and accumulation process for despreading 1 ms or an epoch of the spread GPS signal is to increase the signal-to-noise in order to be able to recognize the GPS data bits. This increase in signal-to-noise that results from the despreading is termed processing gain. Additional processing gain can sometimes be achieved by correlating and accumulating several epochs of the PRN code.
When signal acquisition is achieved the GPS receiver monitors the GPS data bits until a hand over word (HOW) at the start of the subframe is recognized. When the HOW is recognized, the GPS receiver reads time of week (TOW) in the GPS data bits in the HOW to learn a GPS-based clock time. A current precise location-in-space of the GPS satellite is then calculated from the GPS-based clock time and the ephemeris information. The code phase of the GPS replica signal is then used to calculate a pseudorange between the location of the GPS receiver and the location-in-space of the GPS satellite. Typically, the ephemeris information is retained in memory in the GPS receiver from a previous operational mode or is determined by reading additional GPS data bits. The geographical location fix is derived by linearizing the pseudorange about the range between the location-in-space of the GPS satellite and the approximate location of the GPS receiver and then solving four or more simultaneous equations having the locations-in-space and the linearized pseudoranges for four or more GPS satellites.
The global positioning system is commonly used for determining geographical location and/or time in commercial applications including navigation, timing, mapping, surveying, machine and agricultural control, vehicle tracking, and marking locations and time of events. Given such wide commercial application, it is clear that GPS receivers provide a good value for many users. However, the global positioning system has been limited in several potential applications because existing GPS receivers are unable to acquire a GPS signal unless the GPS signal has a relatively clear line of sight to the GPS satellites ensuring strong GPS signals. Typically, this is not a problem where the GPS receiver is mounted on a platform such as a ship, airplane, farm tractor, or a vehicle traveling on an open highway. However, the signal strength requirements of GPS receivers make it difficult to use GPS indoors or where the GPS signal may be weak due to the attenuation of passing through buildings or trees.
In order to increase the strength of the GPS signal within the GPS receiver, workers in the art use techniques for increasing the processing gain above the standard processing gain that occurs by despreading a single epoch of the 1 ms PRN code. For example, the additional processing gain for integrating (correlating and accumulating) ten coherent epochs is 10 log1010=10 decibels (dB) and the increased processing gain for one-hundred coherent epochs is 10 log10100=20 decibels (dB). It would seem that one should increase the number of despread epochs indefinitely until enough processing gain is achieved for overcoming the GPS signal attenuation caused by buildings and trees. Unfortunately, every 20 ms the C/A PRN code may be inverted with a new GPS data bit, thereby nullifying the processing gain for integration times beyond 20 ms. Accordingly, there continues to be a need for improvements in GPS receivers and methods for acquisition of weak GPS signals.